Iris yellow spot virus (IYSV; genus Tospovirus, family
Bunyaviridae) is an economically important pathogen affecting both bulb and
seed onion crops in the US and several parts of the world (1). New reports of
IYSV continue to appear from different states in the US and different countries
in the world. Most recently, IYSV was confirmed in Michigan (2) and Ontario,
Canada (3).

In January 2008, two commercial onion fields in Yuma Co., AZ, had
plants with symptoms suggestive of IYSV infection. Both fields were started from
bulbs and were being grown for seed. At the time of sampling, the plants were
producing flower heads, but the flower heads were immature and had not opened.
Disease incidence in the field located in the southwestern part of the county
was approximately 25%, whereas the incidence in the other field (56 km northeast
of the first field) was in the range of 1 to 2%. Symptoms on leaves included
diamond-shaped or near-diamond shaped lesions. Lesions were as few as one per
leaf or more than six per leaf. Leaves with several lesions showed yellow
streaks due to the coalescing of individual lesions. Diamond-shaped lesions were
more pronounced on scapes compared to those on leaves (Fig. 1).

IYSV infection was confirmed by ELISA using a commercially available kit
(Agdia Inc., Elkhart, IN). The virus infection was further verified by reverse
transcription-polymerase chain reaction. Total nucleic acids were prepared from
symptomatic tissue and primers specific to the small (S) RNA of IYSV (5'-TAA AAC
AAA CAT TCA AAC AA-3' and 5'-CTC TTA AAC ACA TTT AAC AAG CAC-3') were used to
amplify ca. 1.2 kb region of the S-RNA that flanked the complete nucleocapsid
(N) gene (4). The amplicon was sequenced which contained the 822 bp N gene.
GenBank searches confirmed the identity of the amplicon as that of IYSV. There
were no significant sequence differences between isolates collected from the two
fields. Cluster dendrogram consisting of the IYSV isolate from Arizona and
selected IYSV isolates from other parts of the US and the world showed that the
Arizona isolate formed a clade with the one from Canada and within the larger
cluster of isolates from North America (Fig. 2).

Fig. 2. Phylogram based on the nucleocapsid protein sequence of the Iris yellow spot virus isolate from Yuma Co., AZ (designated in the tree as ‘Arizona’). A Neighbor-Joining consensus tree was generated based on 1000 boot strap replications.

Reducing the impact of IYSV requires the adoption of multiple tactics in an
integrated disease management program. Some of the tactics that seem to reduce
the final incidence include ensuring adequate plant vigor by avoiding stressors
such as drought and poor soil fertility (1). Maintaining uniform plant stand,
managing thrips populations and weed hosts, separating bulb and seed crops to
avoid the green bridge, observing field sanitation practices such as reducing
the volunteer onions could also help manage the disease.